Local anomaly in magnetic and electric field variations due to a crustal resistivity change associated with tectonic activity.

Abstract

Electromagnetic response is examined for a local three-dimensional resistivity anomaly embedded in the earth's crust of uniform resistivity , in order to interpret time-dependence of horizontal amplitudes as well as transfer functions for shortperiod geomagnetic variations at an observation site in the Izu region, Japan. The results of numerical estimation indicate that the detected enhancement of about ten percent in horizontal amplitude can be well explained by a local decrease, amounting to one order of magnitude, in crustal resistivity, on the condition that the observation site is located over the area of resistivity decrease. In this situation no marked change is expected for transfer functions; this is consistent with the observation . The results of numerical estimation also suggest that electric as well as magnetic field measurements are useful for detecting crustal resistivity changes; for instance , changes are expected in the amplitude of induced electric field, as would be reflected in apparent resistivity changes in magnetotellurics, and also in polarization angle of the electric field. It should be noted, however, that no detectable precursory changes are expected unless observation sites are located close to the area of resistivity change presumably corresponding to an earthquake source area and its vicinity.

Modeling Low-frequency Magnetic-field Precursors to the Loma Prieta Earthquake with a Precursory Increase in Fault-zone Conductivity

“…Nevertheless, it remains appropriate to consider models in which variations in fault-zone conductivity generate the precursory magnetic-field variations because changes in conductivity do not necessarily require fluid influxes (MORRISON et al, 1977;this paper), and because precursory conductivity changes have been reported before at least two earthquakes on the San Andreas Fault (MAZZELLA and MORRISON, 1974;PARK, 1991). In this paper we demonstrate how more realistic fault-zone geometries than those used by RIKITAKE (1976b) and HONKURA and KUBO (1986) can cause field enhancements by two orders of magnitude, and can be used to successfully model the observed precursory magnetic-field anomalies over a frequency range of three orders of magnitude. The lack of precursory strain change to the Loma Prieta earthquake (less than a few nanostrain at 40 km distance: JOHNSTON et al, 1990) is suitable to our hypothesis because we model the required conductivity change by changing geometry of existing fluid-filled porosity, rather than creation of new porosity.…”

“…The models of RIKITAKE (1976b) and HONKURA and KUBO (1986) are based on the concept of fluid infiltration into a dilatant volume. This concept fell out of favor with the recognition that significant fluid influx requires significant-and easily measurable but rarely observed-precursory strains and uplift (HANKS, 1974).…”

Modeling of seismo-electromagnetic phenomena

“…Therefore we will not consider such sources here, although under special circumstances (e.g. the geometry of local conductivity changes) they may produce anomalies of substantial magnitude [Honkura and Kubo, 1986;Merzer and Klemperer, 1997;Rikitake, 1976aRikitake, , 1976b.…”

Case‐Control Study on a Decade of Ground‐Based Magnetometers in California Reveals Modest Signal 24–72 hr Prior to Earthquakes

“…A broad description of these are discussed by Conti et al (2021)-here we focus only on classical mechanisms that have been invoked to explain ULF band precursors. More detailed discussion can be found in Text S8 in Supporting Information S1, but to summarize, we looked briefly at four common mechanisms and ruled two out (piezomagnetism, and induction within a conductive nucleation zone), but found that at least two mechanisms in the literature can produce the amplitude of effects we observe: (a) Electrokinetic models (Fenoglio et al, 1995;Honkura & Kubo, 1986;Mizutani et al, 1976), which combine the dilatancy hypothesis (Nur, 1972) with the phenomenon of streaming potentials (Onsager, 1931) in the earth, and potentially episodic, propagating fractures (Byerlee, 1993). Interestingly, streaming potential laboratory experiments in rocks (Jouniaux & Pozzi, 1997) produced anomalous signals in the 2-9s frequency band (where our coefficients are most sensitive).…”